Martial arts is a broad term for combative training methods including Judo, Karate, Kung-fu, etc. It also includes the use of various oriental and asian weapons; for example the Samurai sword, the quarter staff, etc. This type of martial-art combat is very old, although, since it is eastern -- or oriental, rather than accidental -- it is just becoming popular in western circles. One of the forms of combat involves a weapon that has two or more elongated, club-like members flexibly connected together in such a way that one member serves as a handle, and the other member -- or members -- serve as a club or cudgel and are free to be swung around the handle to engage a combatant.
For maximum dexterity, the flexible coupling between the two or more members must be as strong and as free-moving as possible so that the handle can swing the other members rapidly and smoothly through a circular arc without any binding or twisting of the coupling. For example, if a chain were used to connect the extremities of two members, the chain links would permit the club member to swing about the handle member, but the chain links would, inevitably, bind or toggle on one another in the course of the rotation.
To accommodate this, conventional swivels have been adapted to one or both ends of the adjacent members to permit more smooth rotation than is possible with the simple chain connection that is rigidly attached at each end. However these swivels usually consist of a form of eye or loop ending in a pin that extends into a hole within which it swivels. This type of swivel is very common and is used on everything from dog leashes to anchor cables, but this type of swivel creates a problem in the distance of the eye or loop above the actual pivot.
This is not important in the case of dog leashes or anchor cables, where the twisting and turning action is almost always along the direction of the rope or cable. However, in the case of a "nunchaku", the strongest pull is at right angles to the joint, and is increased by the centrifugal force of the weapon member being swung around the handle. This lateral pull has a large amount of friction that weakens and wears the swivel joint, giving it a relatively shorter life, a greater possibility of failure, and more chance of binding in use.
In addition, the retaining cap on the handle, for example, covers the head of the swivel shank, so there is no way to see the inevitable wear on the head of the swivel shank and have any warning as to when it is worn to a dangerous degree. This type of swivel is also different to disassemble or replace.